VARIATION IN COLOUR. 
89 
derived tlierefroni, the shells found in other portions of the pool 
being (piite normal in this respect ; and Mr. Ileathcote noticed that 
during the long drought in 1887, when the Ringley Canal was very 
low and the surface covered by an iridescent scum from the aniline 
dye-works, that all the Limnaa stagiudis in tliat locality were remark- 
able for a noticeable metallic gloss and were much darker in colour, 
and more strongly striated than normally ; but the specimens collected 
in the same [ilace when the canal was high and clear were characterized 
by the usual delicate pale horn colour and smooth texture of ordinary 
individuals. 
All aquatic shells whether univalve or bivalve are at times and in 
certain localities entirely or partially covered externally with a reddish 
tint; this is usually owing to living in ferruginous waters and is 
caused by an extraneous deposit of oxide of iron. Analogous incrusta- 
tions of tufa, carbonaceous or other mineral and confervoid matters, 
give to the exterior of aquatic shells different tints, in accordance 
with the nature of this superficial investment, and have given rise 
to some unnecessary varietal names. 
The peristome is sometimes identical in colour with the general 
surfiice of the shell, but often, especially in those species with 
thickened or distinctive lip structure, the colour is strikingly 
different, as in the brown or horn-coloured shells which usually have 
the peristome pure white, occasionally tinted with a delicate rose 
colour. In other species, as HelU' iiemoralis, the colour of the lip 
is very variable and its .special tint may overspread not only the 
umbilical callosity, but the surface of the penultimate whorl, and 
oven extend for some distance into the interior of the shell. This 
lip colouring, especially if not an ordinary characteristic of the 
species, is often very fugitive in character and soon fades after the 
death of the animal. Draparnaud has observed that the peristome 
of Helix insami may become of a bright rose colour if the animal be 
kept without food for a length of time, and the same result is .said to 
occur if the animal lives in warm and sunny places, this colouring 
being .said to be deficient in those individuals living in less favoured 
spots. 
Albinism {nlbus, white) is generally owing to pathological causes 
and is therefore strictly speaking a morbid condition of the animal, 
consequent upon the impaired secretive power or abortion of the 
colour glands, and their consequent physiological inability to secrete 
